As a method replacing a conventional radiography, radiation image recording and reproducing method utilizing a stimulable phosphor is proposed and has been practically employed. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the light emission to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The radiation image storage panel thus treated is subjected to a step for erasing a radiation image remaining therein, and then is stored for the next radiation image recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at considerably smaller dose, as compared with the conventional radiography using a combination of a radiographic film and a radiographic intensifying screen. Further, the method is very advantageous from the viewpoint of conservation of resources and economic efficiency, because the radiation image storage panel can be repeatedly used in the method, while the radiographic film is consumed for each radiographic process in the conventional radiography.
The stimulable phosphor gives a stimulated emission (i.e., light emission) when it is irradiated with stimulating rays after it is exposed to radiation. In a general radiation image storage panel, a stimulable phosphor giving a stimulated emission of a wavelength in the range of 300 to 500 nm when it is irradiated with stimulating rays of a wavelength in the range of 400 to 900 nm is employed. One of the typical examples of such generally used stimulable phosphors is a rare earth activated alkaline earth metal fluorohalide phosphor.
The radiation image storage panel employed in the above-described method generally comprises a support and a stimulable phosphor layer provided on one surface of the support. However, if the phosphor layer is self-supporting, the support may be omitted.
As the stimulable phosphor layer, there are known not only a phosphor layer comprising a binder and a stimulable phosphor dispersed therein but also a phosphor layer composed of only an agglomerate of a stimulable phosphor, containing no binder, which is formed through deposition process or firing process. Besides them, a phosphor layer in which voids of stimulable phosphor agglomerate are impregnated with a polymer material is also known. In any of the above-described phosphor layers, the stimulable phosphor emits light (stimulated emission) when excited with stimulating rays such as visible light or infrared rays, after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiating from an object is absorbed by the phosphor layer of the panel in an amount proportional to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
A transparent protective film of polymer material or deposited inorganic material is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
A radiation image storage panel employing the above-mentioned rare earth activated alkaline earth metal fluorohalide phosphor exhibits high sensitivity and gives a reproduced radiographic image with high sharpness, and therefore the stimulable phosphor is very suitable for practical use. However, according as the radiation image recording and reproducing method is getting more practically used, it is desired to further develop stimulable phosphors of high performance.
U.S. Pat. No. 5,534,191 proposes that the conventional rare earth activated alkaline earth metal fluorohalide stimulable phosphor has some problems caused by the fact that the phosphor is in the form of plate-shaped particles. In order to improve the stimulable phosphor, the U.S. Patent proposes a rare earth activated alkaline earth metal fluorohalide stimulable phosphor in the form of tetradecahedral particles represented by a certain elemental composition formula. In the publication, a radiation image storage panel employing the stimulable phosphor of tetradecahedral type is also disclosed.